As a result of the constant improvement of highway vehicle equipment to increase safety and comfort, the number of electrically controlled devices in motor vehicles has increased significantly. Accompanying this trend, there has been a corresponding significant increase in vehicle cabling. In addition to the increased complexity of the wiring, the wiring harnesses have reached a size which limits their manipulation, and leads to problems with the current compact design of vehicles. In addition to the space requirement, there has also been an increase in the copper requirement and the inaccessibility of the equipment. The increasing number of lines required takes on special importance because, for reasons of mechanical strength, there is a minimum cross-section of the individual lines which must be respected, although in some cases, the measurement or control currents flow only in the milliampere range.
In order to operate with fewer lines, it is already known, e.g., U.S. Pat. No. 3,651,454 and German No. DE-OS 24,33,025, that instead of separate lines, a single multiplex ring circuit can be provided to which all of the control devices are connected by means of branch lines. This ring circuit can, for example, consist in the simplest case of one power feed and one signal line. Switching commands are applied to the signal line in coded form by means of a central transmitter. By means of receivers, which correspond to the devices, the commands are decoded and executed. The execution of the command as well as additional data concerning the actual status of the device, or other information, e.g., analog quantities, are reported back to the transmitter. It is therefore customary for the central transmitter to also receive signals, and for the location receiver to also transmit signals. In order to reduce the number of receivers, several controlled devices are connected to a common transmitter-receiver (DE-OS No. 24,33,025).
Experience has shown that the cause of problems in the electrical system of a motor vehicle is frequently to be found in the wiring, e.g., a result of short circuits or breaks. Such problems have particularly negative effects in the multiplex system with ring circuits described above. It leads not only to the failure of one controlled device, but all or a large number of them. The failure of the central transmitter has the same effect. The known multiplex systems can thus be considered overall as more sensitive to interference than conventional wiring with individual leads.
Accordingly, an object of our invention is a multiplex cabling system for vehicles with less susceptibility to interference and failure than known systems and with reliability equivalent to a direct wire arrangement.
Another object of the invention is a multiplex cabling system to control electrical devices throughout a motor vehicle with a central apparatus for transmitting command messages and receiving indication messages and area transmitter-receivers at each of selected locations to selectively receive the command messages, apply the commands to the corresponding controlled devices, and transmit indications of apparatus status and other conditions to the central apparatus.
Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims when taken in connection with the accompanying drawing.